Major depressive disorder (MDD) is the leading cause of disability in the developed world [85] and is associated with lost productivity, increased health care utilization, and mortality [86]. Several empirically supported treatments, such as antidepressant medication and cognitive behavioral therapy, exist for the treatment of MDD; however, studies have shown that as many as 34% of individuals do not respond to these treatments [5]. Moreover, the large-scale STAR-D trial has shown limited efficacy for combined or sequential treatment with antidepressants and CBT for patients failing to respond to initial interventions [7], with calls for the development of alternative treatments [8]. Exercise interventions represent an alternative strategy associated with strong effect sizes in past studies. For example, a meta-analytic review of exercise programs as compared to non-active conditions for the treatment of MDD found a large overall between-group effect size indicating a significant advantage of exercise interventions for mood benefits [10]. Additionally, exercise has been shown to enhance cognitive functioning, especially attention and memory [40, 41]. This enhancement, as well as the mood benefits derived from exercise, may be due to increases in brain-derived neurotrophic factor (BDNF). Since depression is associated with low levels of BDNF, and antidepressant medication appears to increase BDNF, BDNF may be a mediator through which exercise exerts its effects [32, 33]. The current application aims to investigate the effect of a combined aerobic exercise and behavioral activation treatment (BA) for MDD. Combined exercise treatments have been shown to be efficacious for the treatment of anxiety [21], but a combined model with the empirically-supported depression treatment of BA has yet to be investigated. BA provides a promising format for combination with exercise due to its focus on adaptive action and activity scheduling [24]. The current study will recruit 32 MDD patients for an active treatment phase of 9 total sessions (60 minutes of BA, 30 minutes of EX or CNTL) of either BA plus exercise (BA+EX) or BA plus a stretching control condition (BA+CNTL). Data regarding physical fitness, attention, memory, and BDNF levels will be collected. Additionally, self-report and clinician-rated items regarding depressed mood and level of functioning will be assessed at each weekly session. Data analysis will primarily investigate the feasibility and acceptability of the combined intervention as well as provide preliminary estimates of the efficacy of the treatment. We will also examine the neuropsychological (i.e., memory and attention) and neurophysiological (i.e., BDNF) correlates of the adjunctive exercise intervention as a prelude to future studies of mechanism. The ultimate goal of this application is to develop the applicant's skills in clinical trials research, with a focus on exercise interventions for mood disorders. This proposed research study in addition to relevant coursework, training, and mentorship will aid in the applicant's goal of becoming a knowledgeable researcher in this area of expertise, with plans for future expansion of this intervention for minority individuals in need